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Acquisition (Pro Forma) (Details) - Convergex Group [Member] - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Business Acquisition [Line Items]    
Revenues $ 733,744 $ 681,655
Net income (loss) $ (84,671) $ (50,516)
Basic (usd per share) $ (2.75) $ (1.68)
Diluted (usd per share) $ (2.75) $ (1.68)